In the semiconductor electronics industry, semiconductor chips can be mounted in a flip-chip-configuration with solder balls commonly known as C4 therebetween. The C4 provides an electrical connection and bond between a chip contact location and a substrate contact location. As chip circuit densities increase there is an increased demand for chip I/O. Moreover, if a very large chip is used having a very high density of circuits, a very large number of I/O are required for electrical communication with the chip. The I/O provide signal, power and ground to the chip. As the chip I/O increases, C4s which are solder bonded to the chip contact locations must be smaller and more closely spaced. Moreover, the height of each C4 must be distributed within a relatively small height tolerance to insure that all of the chip contact locations will be electrically interconnected to a corresponding substrate contact location through the C4 bonded therebetween.
Analogous requirements for electronic connections and solder technology also occur at several levels in electronic packaging. This includes solder connections between 1st and 2nd level packages, or 2nd and 3rd level packages. One example is as follows. To connect between a few chips and a large printed circuit board, there is a small printed circuit board, called an interposer. This requires solder connections both between the chip and interposer, and between the interposer and large circuit board. Another example is as follows. To connect between one or a few chips and a large printed circuit board, there is a ceramic package. This uses wire bonds from chip to package, and an area array of solder connections between package an printed circuit board. This ceramic package and chips is sometimes referred to as a hybrid monolithic module. An exemplary list of workpieces suitable for the present invention are: a chip, a substrate, a small printed circuit board, a large printed circuit board, various other electronic packages for various levels, or other electronic device requiring solder connections. Exemplary workpieces include components made of various materials, including but not limited to: semiconductor, ceramic, organic, metallic, crystalline, polycrystalline, polymeric, glassy or amorphous materials.
It is conventional to deposit C4s onto contact locations on a chip or substrate by disposing over the chip or substrate a mask having apertures therein which correspond to the locations of the contact locations on the chip or substrate. The combination of mask and chip are placed into an evaporator which is evacuated. The evaporator contains source materials such as a Pb and Sn. The source is heated to create a vapor of material which deposits upon the mask, filling the apertures therein, and deposits onto the exposed contact locations. Unfortunately, evaporation of C4s do not create very uniform solder heights. Therefore, when there is a very high density of C4s there is a substantial likelihood that a portion of the C4s will not properly electrically interconnect a chip and substrate. Furthermore, solder is evaporated and condensed everywhere in line of sight of the source, leaving orders of magnitude more solder than desired at the pad locations.
Depending on the low vapor pressure and slow deposition rate of the component metals, evaporation may be very time-consuming, and may expose the workpiece to a long time at elevated temperature. The process duration is further increased by the time to load and unload the process vacuum chamber. Injection molding solder has distinct toxicological advantages compared to sputtering or evaporating solder through a stencil. Many solders include lead or other metals which could cause human and environmental damage if they are not contained. During injection molding, almost all the solder is directly and usefully applied to the substrate or other product. There is very little wasted solder, and that is contained in the injection molding system, and can be immediately reused for injection molding. Also solder vapors are fully enclosed and minimized. By sharp contrast, sputtering or evaporation spreads a vastly larger amount of solder throughout a process chamber. Only a small fraction penetrates the mask, and is usefully applied to the substrate or other product. To clean the process chamber and to control the vast excess solder, raises questions of worker safety during cleaning, and environmental safety during disposal or recycling.
It is an object of the present invention to provide an apparatus to deposit onto a substrate solder mounds having substantially uniform height.
The apparatus of the present invention does not require that the workpiece be placed in a vacuum. The apparatus of the present invention deposits solder onto a substrate without requiring that the workpiece be placed in a vacuum.
Moreover, using the apparatus of the present invention, a solder bond having higher aspect ratio (mound height/mound diameter) can be easily fabricated by using an injection mold of appropriate shape.
Moreover, solder mounds having a spatially graded solder composition can be readily fabricated by injecting into a solder mound sequentially solders of different melting temperatures. Thereby, a solder mound can be fabricated which has a relatively low melting point solder at the apex as compared to the solder at the base. If the base is connected to a chip contact location, the solder mound can be heated to the melting temperature of the solder at the mound apex for joining to a contact location on a substrate. By heating only to a melting temperature of the solder of the apex the entire solder mound will not melt and therefore will retain its high aspect ratio shape. High aspect ratio solder mounds provide more bendability. This can be used to avoid failures caused by a thermal coefficient of expansion mismatch between a semiconductor chip and a substrate to which the chip is mounted in a flip-chip-configuration.
Moreover, quite surprisingly it has been found that when molten solder is injected directly onto a polymer surface, the solder mound when solidified adheres to the surface of a polymer. Therefore, a solder mound transfer support surface can be fabricated from a polymer film without an additional adherent layer. An array of solder mounds can be disposed onto the polymer surface using the apparatus of the present invention. The polymer film can then be disposed over a surface having contact locations thereon aligned to the array of solder mounds on the transfer film. Heat is applied which results in the solder mounds wetting the contact locations. The solder is cooled to solidify it. The polymer film is then peeled away leaving the solder mounds on the substrate contact locations. The polymer films peels away since the adhesion of the solder mounds to the polymer film is less than the adhesion of the solder mound to the contact location.
These and other objects, features and advantages will become more apparent from the following, more detailed description, the appended drawings and the appended claims.